1. Field of the Invention
The present invention relates to a switch system used in switching among a plurality of information processing apparatuses for holding data communications. More specifically, the present invention relates to a switch system that employs a loop architecture based on a ring topology.
2. Description of the Related Art
Conventionally, there is known an arrangement of a switch system for controlling data flow in a communication network so as to efficiently process a large amount of data using network apparatuses connected to the communication network. The switch system includes a plurality of input and output terminals for transmitting and receiving data to and from a plurality of network apparatuses. Types of the switch system are classified according to differences in connection form of the input and output terminals. Connection forms of the input and output terminals can be classified into a plurality of forms, i.e., network topologies. Namely, they are classified into a star (tree) topology, a bus (branch) topology, and a ring (loop) topology.
The star topology is an arrangement in which all nodes are connected to one node called “hub”. Examples of a switch system of this type include a crossbar switch in which input and output terminals connected to respective nodes are mutually connected. The bus topology is an arrangement in which a plurality of nodes are connected to a common bus in parallel, and an arbiter that controls occupation of the common bus is added to the bus topology. A star switch and a bus switch that adopt these respective topologies differ in internal structure but can employ protocols of the same format or physical cables of the same type for external network apparatuses.
The ring topology is an arrangement in which a plurality of nodes are connected to one another to form a loop as shown in FIG. 6. FIG. 7 is a block diagram of an internal structure of a ring switch that adopts the ring topology. Each input and output terminal of the ring switch has a pair of connection lines for inputting and outputting data to and from a network apparatus connected to the terminal. The input and output terminals confirm whether or not the respective nodes are present, and a loop is formed by connection between the terminals and external nodes if the nodes are present, and the loop is closed by short-circuiting signal lines if the nodes are not present.
Further, the same protocol is used in the loop. However, in consideration of possible use of a different protocol outside the loop, for example, a switch system can adopt a configuration as shown in FIG. 8. FIG. 8 shows the switch system that enables connection to network apparatuses using a non-ring type protocol by interposing a protocol converter between each input and output terminal and each network apparatus.
Examples of a physical layer protocol applied to the ring switch include a fiber channel-arbitrated loop (“FC-AL”) protocol. The FC-AL protocol is one obtained by expanding a fiber channel (“FC”), which is a serial interface standard, to the ring topology. By executing an arbitration operation among nodes on the loop, a node that is to transmit data to the loop is determined. In this arbitration operation, the data is transmitted and received upon completion of preparations by a transmission end and a reception end.
Examples of a conventional switch system using the FC-AL protocol include a switch system disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 2000-261498. JP-A No. 2000-261498 discloses a method for appropriately setting an arbitrated loop physical address (AL_PA) used for the FC-AL to hardware.
The conventional technique has, however, the following disadvantages. If the FC-AL protocol is applied to the switch system, only one packet (which is referred to as “frame” under the FC-AL protocol) can be transmitted into the loop. Due to this, if the FC-AL protocol is used, then a communication band is used inefficiently, and it is difficult to improve multiplexing of data communication.
Furthermore, if the FC-AL protocol is used, it is necessary to travel a signal for the arbitration operation around the loop. Namely, unless time corresponding to one round of the loop passes, no nodes can be used. As a result, a delay of a half round of the loop is generated on average. Thus, it is difficult for the ring switch using the FC-AL protocol to establish high-speed communication, as compared with the star switch in the topology in which the respective nodes are connected to one another.